1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic process, an electrostatic recording process etc.
2. Related Background Art
(1) Image Forming Process
An image forming apparatus is generally provided, as shown in FIG. 8, with a photosensitive drum 10 constituting a latent image bearing member, a charging apparatus 11 constituting charging means which uniformly charges the photosensitive drum, an exposure apparatus 12 for applying an imagewise exposure to the uniformly charged photosensitive drum thereby forming an electrostatic latent image, a developing apparatus 13 for developing the electrostatic latent image with a toner, constituting a developer, thereby obtaining a visible toner image, a transfer apparatus 15 constituting transfer means which transfers the toner image, present on the photosensitive drum, onto a transfer material 14 constituting a transfer medium, a fixing apparatus 16 for fixing the toner image on the transfer material, and a cleaning apparatus 17 constituting cleaning means which scrapes off toner remaining on the photosensitive drum 10. The photosensitive drum 10, the charging apparatus 11, the developing apparatus 13 and the cleaning apparatus 17 are often constructed as a process cartridge, detachably mounted on a main body of the image forming apparatus.
The image forming apparatus executes an image formation by repeating the steps of charging, exposure, development, transfer, fixation and cleaning with the above-mentioned means.
(2) Operation Sequence of Image Forming Apparatus
FIG. 9 shows a general operation sequence of an image forming apparatus.
When a detachable process cartridge is inserted into a main body of the image forming apparatus and a power supply therein is turned on, a main motor is activated to initiate an initial multi-rotation step. This step executes a detection of presence/absence of the process cartridge, and a cleaning of a transfer roller (toner attached on the transfer roller being discharged onto the photosensitive drum).
After the initial multi-rotation step, the image forming apparatus moves a stand-by state. When image information is supplied from output means such as an unillustrated host computer to the image forming apparatus, the main motor drives the main body of the image forming apparatus thereby entering an initial rotation step. This step executes preparatory operations for printing in various process devices, principally including a preliminary charging of the photosensitive drum, a start-up of a laser scanner, a determination of a transfer voltage in the image formation, and a temperature regulation of the fixing apparatus.
After the initial rotation step, an image forming step is initiated, including a supply of a transfer material at a predetermined timing, an imagewise exposure on the photosensitive drum, a development, a transfer, a fixation etc.
After the image forming step, in case a next print signal is present, there is entered an intersheet step for awaiting a next printing operation until a next transfer material arrives. In case of absence of a next print signal, the image forming apparatus enters a post-rotation step, which executes a charge elimination of the surface of the photosensitive drum, and a cleaning of the transfer roller.
When the post-rotation step is completed, the image forming apparatus enters a stand-by state again, thus waiting for a next print signal.
(3) Charging Apparatus and Control Method for Charging Bias Voltage
For the charging apparatus 11, there is widely employed a contact charging method of maintaining a charging apparatus of a roller or blade shape into contact with the surface of the photosensitive drum and applying a voltage to the charging apparatus thereby charging the surface of the photosensitive drum. In particular, the charging method of roller type can achieve a stable charging over a prolonged period.
A charging bias voltage source applies a charging bias voltage to the charging apparatus. The charging on the photosensitive drum may be achieved by a charging bias voltage constituted solely of a direct current voltage, but there is generally employed a bias voltage, as disclosed in Japanese Patent Application Laid-open No. 63-149669, formed by superposing a direct current voltage Vdc corresponding to a desired dark potential Vd on the drum with an alternating current voltage having a peak-to-peak voltage (Vpp) equal to or higher than two times of a discharge starting voltage under a direct current voltage application. (In the following, a direct current is represented by DC, an alternating current is represented by AC, and the above-described charging method is represented as AC+DC charging.)
This charging method is suitable for uniformly charging the surface of the photosensitive drum 10. By superposing the DC voltage with an AC voltage equal to or higher than a certain level, a local potential unevenness (charging failure) on the photosensitive drum is eliminated by a leveling effect of the AC voltage, whereby a charged potential Vd on the surface of the photosensitive drum uniformly converges to DC voltage Vdc.
The AC+DC charging is characterized in having a larger discharge current to the photosensitive drum, in comparison with a DC charging in which a DC voltage alone is applied. With an increase in the discharge current to the photosensitive drum, a chain connecting molecules on the surface of the photosensitive drum tends to become more easily cleavable. Consequently a resin constituting the surface of the photosensitive drum is modified toward a lower molecular weight, and becomes more easily scrapable with a cleaning blade. Therefore the surface of the photosensitive drum is polished and can enter a next image formation (charging step), even after repeated use, in a refreshed state as in an initial stage of use without a surface contamination for example by a transfer residual toner.
However, in case an excessive discharge current continues to be applied to the surface of the photosensitive drum, a surface layer of the photosensitive drum is scraped off with a higher speed, whereby the photosensitive layer of the photosensitive drum reaches a limit film thickness where the photosensitive layer can no longer exhibit its function in an early stage after the start of use, thus coming to the end of the service life. Upon reaching such limit film thickness, the photosensitive layer loses its function, thus exhibiting a small unevenness in the charging, or generating a charging failure as a result of a loss in the charge holding ability of the surface. In the actual use, therefore, the discharge current to the surface of the photosensitive drum has to be so regulated as not to become excessively large.
A relation between a peak-to-peak value Vpp of the AC voltage and the discharge current is not constant but is influenced for example by an environment of use (a change in the impedance of the charging roller), a thickness of a charge transport layer of the photosensitive drum etc. For example, even under an application of an AC voltage of a constant peak-to-peak value Vpp, the discharge amount decreases in an environment of a low temperature and a low humidity because of an increase in the impedance of the charging roller, and increases in an environment of a high temperature and a high humidity because of a decrease in the impedance of the charging roller. Also under a same environment of use, when the surface of the photosensitive drum is scraped off by the cleaning blade during the use, the discharge amount increases because the impedance becomes lower than at the initial stage of use.
In order to avoid such drawback, U.S. Pat. No. 5,420,671 proposes a method of controlling the AC component with a constant current. This method is to detect an AC current Iac from the charging roller to the photosensitive drum and to control such current at a constant level, and can maintain the discharge current substantially constant since the peak-to-peak value Vpp of the AC voltage changes flexibly in response to changes in the impedances of the charging roller and the photosensitive drum. This method is very effective in securing a satisfactory charging property and preventing an excessive discharge to the photosensitive drum.
This method requires, however, in order to obtain a stable bias voltage, to separate power supplies for the AC and DC components to be superposed, thus necessitating two voltage-elevating transformers. Within a power supply circuit, a voltage-elevating transformer is a component relatively large and relatively costly. For this reason, particularly in a compact and low-cost image forming apparatus, it has been desired to realize a stable charging bias voltage utilizing single voltage-elevating means, not dependent on an environment of use or of a thickness of the photosensitive drum, thereby providing the photosensitive with a stable discharge current.
Therefore, it is proposed, as described in U.S. Patent Application Publication No. 2003219268, to provide a stable discharge current by a charging bias supply circuit involving single voltage-elevating means, not dependent on the environment of use. Such a configuration will be explained in the following.
FIG. 10 is a schematic view of a charging bias supply circuit. It is based on a constant voltage control having plural AC oscillation outputs (Vpp-1, Vpp-2, . . . , Vpp-n; wherein peak-to-peak voltages have a following relation Vpp-1>Vpp-2> . . . >Vpp-n> . . . ), and utilizes only one voltage-elevating transformer for generating an AC component, and a DC is generated by a peak charging of a capacitor C10 by such voltage-elevating transformer.
An engine controller applies, from such AC oscillation outputs, the AC voltages with plural peak-to-peak voltage Vpp, and selects, as a peak-to-peak voltage of the charging AC voltage at the image formation, such a minimum Vpp that provides an AC current Iac in the photosensitive drum 10 equal to or larger than a peak-to-peak voltage selection control threshold current Iac-0 required for a charging AC voltage not inducing a charging failure.
Such charging bias voltage control allows a substantially constant current behavior to be obtained, as in a constant current control, independent from a change in the impedance in the charging roller, the photosensitive drum etc.
Such a charging voltage control method will be called a peak-to-peak voltage selection control.
(4) Elimination of Foreign Substance on Photosensitive Drum
As explained in the foregoing, the surface of the photosensitive drum is maintained, even after repeated use, in a refreshed state equivalent to an initial state by polishing with a cleaning blade, and can enter a next image formation (charging step) without a contamination for example by a transfer residual toner.
A foreign substance such as the transfer residual toner is usually scraped off in a post-rotation step after an image formation. However, if a deposited foreign substance is in a state not easily separable from the surface of the photosensitive drum, a polishing in the post-rotation step and an initial rotation step in a next job may be insufficient for removing the foreign substance. A printing process executed with an uneliminated foreign substance may result in an image defect resulting from such a foreign substance. A following phenomenon is an example of such situation.
Referring to FIG. 11, after an end of an image forming process, a foreign substance 19 such as a transfer residual toner or a power scraped off from the photosensitive drum is positioned between the cleaning blade 17 and the photosensitive drum 10, and is pressed to the photosensitive drum 10 by the pressure of the cleaning blade 17, thus becoming not easily separable. A position X on the photosensitive drum where the foreign substance is deposited becomes different in a friction coefficient, in comparison with other positions (free from the foreign substance) on the photosensitive drum. When a next image forming process is initiated in this state and the position X reaches the cleaning blade 17 after one turn, the rotating speed of the photosensitive drum 10 becomes different only in the position X since it is different in the friction coefficient in comparison with other points. Therefore an exposure blur is generated in an exposure position Y, leading to a white streak image uniform in the longitudinal direction, as shown in FIG. 12. Then, when this position again reaches the position of the cleaning blade, a same phenomenon is repeated whereby white streak images are generated at a period R corresponding to a peripheral length of the photosensitive drum.
Since the deposited foreign substance 19 is scraped off little-by-little by the cleaning blade 17, the white streak image is most conspicuous in a first print where the amount of the foreign substance is largest, then, in a continuous use, becomes progressively less conspicuous in a second print, a third print and so forth since the foreign substance is gradually scraped off, and eventually vanishes completely as the foreign substance is eventually removed completely.
Therefore, this phenomenon can be resolved by extending a rotation time of the photosensitive drum prior to the image formation. An extension of the rotation time of the photosensitive drum before the image formation increases the chance that the position with a deposited foreign substance passes under the cleaning blade, thereby completely eliminating the foreign substance eventually.
However, in case of executing a peak-to-peak voltage selection control for the charging AC voltage and extending the photosensitive drum rotation time for completely eliminating the foreign substance from the photosensitive drum, there is required a longer time before the image formation and the time required for the entire printing process results in a significant elongation, which is undesirable from the standpoint of usability.
The present invention is to solve the aforementioned drawbacks.